16 A Challenging Diagnosis of Venous Malformation and Diffuse-type Tenosynovial Giant Cell Tumor Occurring in the Knee of a 9 Year-Old Male

Introduction/Objective Diffuse-type tenosynovial giant cell tumor (TGCT), previously known as pigmented villonodular synovitis, is a rare pathology especially in young individuals. The symptoms are usually of relatively long duration, and the wide differential diagnosis can result in delays in diagnosis. Methods/Case Report We present a case of a 9-year-old male with recurrent left knee pain and swelling, initially diagnosed with a venous malformation based on an initial biopsy. Immunohistochemistry and TEK gene mutation testing were performed to confirm the venous malformation diagnosis. However, after surgery, a second pathology of diffuse-type TGCT was also identified. An initial tissue diagnosed as vascular malformation was investigated with immunohistochemistry using antibodies against CD34 and D2-40. It was sent for molecular identification to an outside institution and for next-generation-sequencing. Results NA Conclusion We suspect that the dual pathology was not recognized earlier due to the limited sampling from the initial superficial biopsy. The definitive diagnosis was made following further investigation, including immunohistochemistry and molecular studies.